The overall goal of this project is to complete the development and to test a novel ultrasonic tapered linear phased array applicator for hyperthermia treatment. Such an applicator will allow electronic steering of the focus along a desired path without the use of a mechanical scanning system and will remain fixed relative to the patient, making coupling easier. The applicator will be computer controlled such that the scan path and/or intensity can be altered at any time during treatment to maintain a constatn uniform tumor temperature. Such a system is expected to meet the requirements for controlled local hyperthermia treatment of deep-seated tumors. Each element of the phased array applicator (64 in all) will be driven by a modular high-power multichannel RF generator similar to that used in the URI Therm-X, Inc. Sonotherm 1000 ultrasound hyperthermia system. Initial testing of the applicator will consist of experimentally acquiring field plots and comparing them to the theoretically predicted plots. Once satisfied that the experimental phased array applicator is generating fields as expected theoretically, and that these fields are such that they should be adequate for hyperthermia applications, tests on phantoms will be initiated. Finally, the performance of the prototype applicator which electronically scans its focus will be compared to that of a mechanically scanned, fixed focus applicator for various scan patterns. If the proposed prototype ultrasonic phased array applicator exhibits heating performance and control commensurate with clinical needs, Phase II development will be devoted to the construction and animal testing of clinical prototype system in preparation for clinical trials.